Field of the Invention
The present invention relates to a removal method of manganese from waste water. More specifically, the present invention discloses a method in which manganese alone can be efficiently removed by selective precipitation from sulfuric acid waste water containing aluminum, magnesium and manganese, while the precipitation of magnesium is inhibited.
Description of the Prior Art
In treating of industrial waste water, it is a crucial technology to separate heavy metals from the waste water, particularly through precipitation (also called fixation), from an environmental conservation perspective. Among the heavy metal pollutants, fixation of the manganese pollutant in the acidic waste water is particularly difficult. Conventionally, a considerable amount of slurry of industrial calcium hydroxide is used as a neutralizing agent, and the pH of the waste water is adjusted to a value in the range of 8.0 to 10.0 to form the manganese precipitate for easy removal. However, applying this manganese removal method to sulfuric acid waste water containing magnesium and manganese turns out to be problematic, because the amount of neutralizing agent for this reaction greatly exceeds the chemical equivalent amount needed for fixing the manganese. This is because the magnesium sulfate precipitates together with the manganese, unnecessarily consuming the neutralizing agent.
To address this problem, a method for preferentially removing manganese from an acidic solution containing both magnesium and manganese is disclosed (see Patent Literature 1). In the method of Patent Literature 1, when treating a manganese acidic solution containing 1 to 3 g/L of Mn and 2 to 15 g/L of Mg, obtained from the acid-leaching of nickel oxide ore, manganese is preferentially precipitated and removed by controlling the addition of air, oxygen, ozone or peroxide, so that the pH of the solution is adjusted to a value within the range of 8.2 to 8.8, and the oxidation-reduction potential (mV) is within 50 to 300 my. According to this method, manganese is oxidized, and the oxidized manganese is more preferentially neutralized than magnesium so as to be removed in a form of precipitate. The pH value for the method of Patent Literature 1 is about 8.0 after neutralization, under which less magnesium precipitates because most magnesium at pH 8.0 exists as magnesium hydroxide, which has good solubility. Hence, the amount of consumed neutralizing agent is less than the conventional method.
However, when the method of Patent Literature 1 is applied to an acidic solution containing aluminum in addition to magnesium and manganese, magnesium co-precipitates with manganese. Therefore, the method of Patent Literature 1 still has the problems of requiring an excessive amount of neutralizing agent, an increased amount of treating time, and co-precipitation of magnesium with manganese. The co-precipitation of Mn and Mg prevents easy utilization of the precipitated manganese.
From the above introduction, it is clear that a method of removing manganese selectively while inhibiting the precipitation of magnesium in treating the sulfuric acid waste water containing aluminum, magnesium and manganese is needed.